Internal Modified Atmospheres of Coated Fresh Fruits and Vegetables: Understanding Relative Humidity Effects L. CISNEROS-ZEVALLOS AND J.M.

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1 Concise Reviews in Food Science JFS: Concise Reviews and Hypotheses in Food Science. Internal Modified Atmospheres of Coated Fresh Fruits and Vegetables: Understanding Relative Humidity Effects L. CISNEROS-ZEVALLOS AND J.M. KROCHTA ABSTRACT: Internal atmosphere modification in fruits and vegetables by surface film coatings depends on film permeability, coating thickness, and fruit surface coverage. Research in this area has been mostly empirical, with unpredictable results and diverse conclusions. To reduce variability, it is necessary to have a better understanding of factors that may influence the response of coatings applied to fruits. One factor is relative humidity (RH), which is known to affect the permeability of biopolymer films. By using steady-state mathematical models as tools, we hypothesize that fruits coated with hydrophilic films will be largely influenced by the RH of storage conditions. For hydrophobic materials, RH would have less influence on coating performance. Keywords: relative humidity, fruits and vegetables, coatings, mathematical models, modified atmosphere Introduction COATINGS APPLIED TO THE SURFACES OF FRUITS AND VEGETABLES can be formed from one or more components and are commonly called waxes (Hagenmeier and Shaw 1992). However, coating materials used are actually various mixtures of lipids, proteins, carbohydrates, plasticizers, surfactants, additives, and solvents like water and alcohols. The different materials used and the properties they possess provide a wide range of characteristics possible for the coating systems. Although research in this area is extensive, much of the work has been empirical (Banks and others 1993). Commercial coatings are used mainly to prevent water loss or to impart gloss to fruits and vegetables. However, coatings have not been widely implemented on a commercial scale for specific atmospheric modification, in part due to the variation in the response of fruits to apparently similar coatings and to uncontrolled factors that influence the coating performance. Different factors that can affect the performance of coating-fruit systems are type of fruit, coating surface coverage, coating thickness and permeability, and temperature. Banks and others (1993) proposed a mathematical model to explain the interaction between a fruit and a coating system. Through this study they tried to explain the variability of results found in the literature. They described coatings performing either as a film wrap or by blocking pores. The first mechanism is called the loosely adhering coating (lac) model. In the lac model, the coating is assumed to function like a film wrap covering the fruit skin, where coating and fruit resistances act in series (Ben-Yehoshua and Cameron1988; Hagenmeier and Shaw 1992). In this model, fruit resistance is the result of pore and cuticle resistance operating in parallel. The modified atmosphere (MA) generated in the coated fruit will depend on coating permeability and coating thickness. The second mechanism is called the tightly adhering coating (tac) model. In the tac model, the coating tightly covers the pores and cuticle, so the pore/coating and cuticle/coating resistances are added in parallel. In this model, pore blockage is more important than coating characteristics. Thus, MA will depend on the differing proportions of pores blocked by the coating. Between the lac and tac models, the real mechanism is still unknown. One factor not considered throughout the literature is the relative humidity (RH) of storage conditions. RH is known to affect the permeability values of hydrophilic films (McHugh and others 1993, 1994; McHugh and Krochta 1994a, 1994b). Usually fruits are stored between 90 and 95% RH; however, values lower than these are common in commercial practice and in research studies. The transmission of molecules through polymer films, known as permeability, is a basic function of the chemical structure and of other factors such as polymer morphology, including density, crystallinity, and polymer orientation. Higher densities, higher degrees of crystallinity, and crosslinking will all decrease the permeability of a polymer film. The type of solvent used in casting films and the drying rate will also influence the permeability coefficient (Pauly 1989). Plasticizers are often added to increase film flexibility; however, this will also increase film permeability (McHugh and others 1994; McHugh and Krochta 1994a; Mate and Krochta 1996). Water is the most effective plasticizer in hydrophilic films, and the amount of moisture in the films is related to the RH of the environment through their moisture sorption isotherm behavior (Gontard and others 1996). Thus, permeability of hydrophilic films and films formed as coatings, increases at higher RH because of increasing moisture concentration within the film (Mark and others 1966; Elson and others 1985; Rico-Pena and Torres 1990; Hagenmeier and Shaw 1991; McHugh and Krochta 1994a). This increase in permeability is also related to a decrease in the glass transition temperature of the film. An amorphous polymer matrix may exist as a viscous glass or a liquid-like rubber state. The change from one state to another will strongly depend on water content or other plasticizers. The glass rubber transition will affect molecular mobility of the system, which can be observed as changes in viscosity, diffusivity, or flexibility of the system (Roos and Karel 1991; Buera and Karel 1993; McHugh and Krochta 1994a). Reports of RH effects on coated fruits are few. Elson and others (1985) reported how RH affected the O 2 and CO 2 permeability characteristics of a hydrophilic coating (Nutri-save) and the resulting 1990 JOURNAL OF FOOD SCIENCE Vol. 67, Nr. 6, Institute of Food Technologists

2 Coated fruits and vegetables and RH effects coating effect on apples. Meheriuk and Lau (1988) related the loss of normal ripening in coated pear fruits stored at low RHs to a reduction in coating permeation. Most of the work of RH effects has been on film permeabilities. For example, Hagenmeier and Shaw (1991) showed that O 2 permeability of shellac coatings was dependent on RH. Coatings cast from ethanol, compared with water, had lower permeability values and were less dependent on RH. Similar RH dependency of gas permeability for fruit-coating waxes was reported (Hagenmeier and Shaw 1992). It was stated that this RH dependence is in large part due to the polar components used to raise ph and solubilize the polymer. Rico-Pena and Torres (1990) found an exponential dependence of O 2 permeability with RH for an edible methylcellulose palmitic acid film. Whey protein films have a similar dependence of O 2 and aroma permeability on RH conditions (McHugh and Krochta 1994a; Miller and Krochta 1997, 1998a; Miller and others 1998). RH dependence on gas permeabilities for amylose starch films (Mark and others 1966) and hydroxypropylated starch films (Roth and Mehltretter 1967) were also reported. More recently, Gontard and others (1996) and Mujica and Gontard (1997) showed the O 2 and CO 2 permeability dependence on RH for wheat gluten films. The objective of this study was to use steady-state mathematical models to understand the influence of RH on the performance of hydrophilic films formed as coatings on fruits, and to explain, in part, the conflictive results found in the literature. Theoretical Approach where, RQ is the respiratory quotient when O 2 is unlimited (= 1), constant a = 0.007, and constant b = 5. (c) Internal oxygen partial pressure, p O2i. At steady state, r O2 = J O2, where J O2 is the diffusive flux of O 2 between internal and external atmospheres per unit weight of fruit. According to Fick s law: A fruit is the fruit surface area (cm 3 ), p O2e is the external oxygen concentration (atm), W is the fruit weight (kg), and R O2 total is the O 2 diffusion resistance between the fruit s internal and external atmospheres (s atm/cm). Resistance values will include skin and/or coating thickness. Combining Eq. 1 and 3 and solving for p O2i : (d) Internal carbon dioxide partial pressure, obtained from Eq. 2 and 3: (3) (4) Concise Reviews in Food Science Fruit model system The fruit model system is analyzed as a modified atmosphere (MA) model system. The MA model is described as a function of the internal atmospheres. In this study, we used the lac model and the tac model for the specific case of complete surface coverage and gas exchange mainly through pores. Gas exchange depends on coating permeability and thickness for achieving internal MA. The assumptions considered for the model are steady-state condition, constant temperature, no effect of CO 2 on respiration rate, and no internal atmosphere composition gradients within the fruit. The model consists of equations that describe fruit respiration rate, diffusion through skin and coating (Banks and others 1993), and the coating permeability dependence on RH. Equations of the fruit model system are those proposed by Banks and others (1993) for apple fruits. However, the approach can be extended to other fruits as well. (a) Fruit oxygen uptake (r O2, cm 3 /kg/s): where V m is the maximum rate of O 2 consumption (cm 3 /kg/ s); K m is the Michaelis-Menten constant (atm), and p O2i is the internal oxygen partial pressure (atm) (Banks and others 1993). K m is the equivalent to the substrate concentration that yields half the maximal rate of O 2 consumption and indicates the affinity of the enzyme system to the substrate. (b) Respiratory CO 2 production (aerobic and anaerobic), (r CO2, cm 3 /kg/s). (1) (2) Here, p CO2i is the internal carbon dioxide partial pressure (atm), R CO2 total is the resistance to CO 2 diffusion between the fruit s internal and external atmospheres (s atm/cm). (e) The lower oxygen limit (LOL) is defined as the p O2i under which fermentation is initiated, being product-specific. LOL is determined when r CO2 /r O2 = RQ > 1.1 in this study, where RQ is the respiration quotient. The LOL is calculated as atm for the fruit model system. (f) For the lac model, the coating resistance (R coat ) and fruit resistance (R fruit ) are added in series. Thus: (5) R total = R fruit + R coat (6) A similar relationship describes the tac model in the case of complete surface coverage and gas exchange mainly through pores. In this latter case, R fruit = R pores. Permeability dependence on RH The coating system used in this study was plasticized, edible whey protein isolate (WPI) film (McHugh and Krochta 1994a) with an exponential O 2 permeability dependence on RH (25 C), defined in this study as: P O2 = j 10 k RH (7) where RH is relative humidity (%), P O2 is O 2 film permeability (cc- m/m 2 -d-kpa), and jand k are constants. For example, values of c for WPI-sorbitol films (WPI/Sorb, ratio 60/40) and WPI-glycerol films (WPI/Gly, ratio 70/30) are and 1.208, respectively. Values Vol. 67, Nr. 6, 2002 JOURNAL OF FOOD SCIENCE 1991

3 Concise Reviews in Food Science Coated fruits and vegetables and RH effects of d for WPI/Sorb and WPI/Gly films are and 0.037, respectively. Coating resistance to O 2 (R O2 coat, s atm/cm) is defined as: R O2 coat = H/n P O2 (8) where H is coating thickness (cm) and n is the conversion factor (n = m 2 d kpa/ m cm s atm) used to obtain P O2 values in cm 2 /atm s. A general assumption for the coating system is that it equilibrates completely with the RH of the external atmosphere (no moisture concentration gradient within the film) at a constant temperature (25 C). It is also assumed that the surrounding air is well mixed; therefore, the bulk air RH is similar to the air RH at the film air interface. The film thicknesses used were 0.75, 1.3, and 2.6 m. The ratio between CO 2 and O 2 film permeability (P CO2 /P O2 ), also known as film value, was assumed to be 2, 4, or 10. It was also assumed that the fruit skin resistance (R fruit ) to gas exchange was not affected by changes in RH. Assumed values for the different variables in this study are as follows: A = 160 cm 2, W = 160 g, K m = 0.02 atm, p O2e = 0.21 atm, R O2 fruit = 5996 atm s/cm, R CO2 fruit = 6535 atm s/cm, V m values used were 5, 20, and 30 cm 3 /kg-h (from Banks and others 1993). By using this steady-state approach, we can study the effects of RH on p O2i, p CO2i, r O2, r CO2, and RQ of coated fruits if we know values for V m, K m, A, W, H, P O2, and R fruit. Results and Discussion RH and coating thickness effects on internal gas pressure of coated fruits Oxygen permeability of whey protein film coatings increases with RH in a exponential form according to Eq. 7 (Figure 1). The same tendency is shown for CO 2 permeability for an assumed value of 4. When a hydrophilic film is formed as a coating on a fruit, the model predicts p O2i and p CO2i change as the external RH decreases (Figure 2). Internal oxygen is reduced and carbon dioxide is increasingly accumulated within the fruit as RH decreases. Thus, storing fruits at a given RH (for example, 65 to 70% RH) can deplete the p O2i inside the fruit and induce a large p CO2i increase. This effect will be influenced by coating thickness. For a given RH, the p O2i within the coated fruit is lowered and p CO2i is increased with an increase in thickness. As a result of this, fruits coated with thick films may reach large p CO2i at higher RH compared with fruits coated with thinner films. Gas exchange will show similar tendency, with a decrease in r O2 and r CO2 when RH decreases. The explanation is that as RH decreases, film O 2 and CO 2 permeance decreases, thereby lowering the p O2i levels within the fruit. Increasing film thickness will reduce even more the O 2 and CO 2 permeances and lower even more the p O2i within the fruit. When a LOL is reached, anaerobic respiration will dominate, inducing large r CO2 production and accumulation of large p CO2i within the fruit at a critical RH. Thus, anaerobic conditions can be reached at different RHs depending on coating film thickness. Although fruit respiration has not been studied as a function of film thickness, investigations have been based on the amount of coating material applied or coating solution concentration used (Ben Yehoshua and others 1970; Banks and others 1993). Film thickness has been related to coating solution concentration (Park and others 1994a, 1994b) and will probably be influenced by physicochemical properties of the coating solution. Coating thicknesses have been reported to range from 1 to 5 m (Trout and others 1953; Brusewitz and Singh 1985; Elson and others 1985; Hagenmeier and Shaw 1992) and from 4.5 to 13 m (Park and others 1994a). Our model predictions may partly explain the variability observed in the Figure 1 Exponential relationship of O 2 and CO 2 permeability with relative humidity (% RH) for a plasticized whey protein film (60% whey protein/40% sorbitol) at 20 C. Figure 2 Predicted p O2i and p CO2i of WPI/Sorb-coated fruits stored under different RH conditions and for different coating thickness. Film b = 4 and film coating thickness are 0, 0.75, 1.3, and 2.6 mm. Fruit V m is 20 cm 3 /kg-h at 20 C JOURNAL OF FOOD SCIENCE Vol. 67, Nr. 6, 2002

4 Coated fruits and vegetables and RH effects past when studying coated fruit systems. Changes in RH and variation in coating thickness can interact, inducing large effects in internal MA of coated fruits. RH and maximum respiration rate effects on internal gas pressure of coated fruits. We considered a fruit model with 3 different V m values (low = 5, moderate = 20, and high = 30 cm 3 /kg-h), which represent fruits with differing V m values or fruits stored at different temperatures. Coated fruits have a constant film thickness. In this case, the model predicts that coated fruits with higher V m would yield lower internal p O2i as RH decreases, compared with coated fruits with lower V m. The trend is similar to the effect observed when film thickness decreases in coated fruits (Figure 2). Similarly, internal p CO2i will be larger for fruits with higher V m values as RH decreases. For all fruits, there is a RH at which a large increase in p CO2i will occur. This critical RH will be lower for fruits with lower V m, and higher for fruits with higher V m. Oxygen uptake, r O2, for coated fruits with differing V m will also be affected by RH in similar fashion. We hypothesize that coated fruits with a large V m will reach the LOL at higher RH values compared with fruits with a lower V m. Even though film permeability declines with decreasing RH, the p O2i, r O2, and r CO2 for fruits with lower V m remains fairly constant down to lower RH values. This makes it possible to store fruits with low V m over a wider range of RH conditions. Fruits with a large V m are severely affected by RH when coated with hydrophilic films. Large decreases in p O2i, r O2, and r CO2 are observed with small drops in RH. This response is due to the higher demand of O 2 by the fruit and the decreasing film gas permeability with lower RH. Therefore, coated fruits with a large V m can be stored only over a short range of RH before anaerobic conditions are reached. The composition of the film coating will influence the moisture sorption properties of the coating material and may cause the dependence of film gas permeability and values on RH (Gontard and others 1996). The trend of hydrophilic films is to have similar moisture sorption properties and gas permeability dependence on RH. However, gas permeabilities of nonhydrophilic films (such as pure lipid films) are not affected by RH since their moisture sorption properties are negligible or independent of RH (McHugh and others 1993). For emulsified wax films (such as commercial waxes), the gas permeability dependence on RH will depend on film composition. For example, an emulsion film may consist of lipid particles dispersed in a hydrophilic matrix, giving a gas permeability dependence on RH between that of a hydrophilic film and a pure lipid film. This gas permeability dependence may also be influenced by lipid particle size, particle arrangement within the matrix, and lipid concentration (Shellhammer and Krochta 1997). Curves similar to those in Figure 3 can also be constructed for coated fruits stored at only one RH. For a coated fruit with a given V m and constant film value, changes in film thickness or film permeability will give points on the same curve. For example, thicker films will give points shifted toward the left, while films with a larger permeability will give points shifted toward the right. This indicates the possibility of tailoring films with appropriate values, thickness, and permeability characteristics to allow specific combinations of p O2i and p CO2i to be reached or to prevent detrimental anaerobic conditions. These p CO2i /p O2i plots can be used as tools to design coatings that can target specific combinations of O 2 and CO 2 for coated fruits (Banks and others 1997). Coating design for fruits with differing V m values would be possible by selecting coating materials with appropriate film values, permeabilities, and thickness, and by controlling RH conditions. Concise Reviews in Food Science Internal CO 2 /O 2 plots as influenced by RH, thickness, maximum respiration rates, and film b values: their importance in coating design Carbon dioxide/oxygen plots for internal gas composition of coated fruits can be used in a similar way to the CO 2 /O 2 plots of external gas composition used for modified atmosphere packaging design (Mannaperuma and others 1989). The latter plots are used to target external p O2e and p CO2e combinations or windows for storing fruits under external modified atmospheres. For coated fruits, we use the p O2i and p CO2i within the fruits, and the target windows would be the combinations of internal gases appropriate for storage. These plots can also give us information on the internal LOL values, above fruits can be stored safely and under which anaerobic conditions are generated. By changing external RH conditions for a coated fruit, we can generate a p CO2i /p O2i plot. Each point of the curve corresponds to a different RH condition. The position of the curve will be influenced by the fruit s V m, producing curves closer to the X-axis for those fruits that have lower V m (Figure 3). The ratio between O 2 and CO 2 permeabilities (P CO2 /P O2 ) also has an effect on these plots. Film values are generally in the range of 3 to 5 for coating materials (Hagenmeier and Shaw 1992). Coated fruits with larger film values will yield curves closer to the X-axis and have smaller slopes, compared with coated fruits with smaller film values. More recently, Gontard and others (1996) have shown that b increases with increasing RH, depending on film type. A generated curve assuming non-constant b values will have similar trend as those curves generated assuming constant values (Figure 3). Figure 3 Predicted p CO2i /p O2i plots, generated by storing WPI/ Sorb-coated and uncoated fruits under different RH conditions and different V m values. Fruit V m values are 5, 20, and 30 cm 3 /kg-h. Film thickness is 1.3 m and film = 4. Vol. 67, Nr. 6, 2002 JOURNAL OF FOOD SCIENCE 1993

5 Concise Reviews in Food Science Coated fruits and vegetables and RH effects Possible sources of variation in internal gas composition One assumption in this model system is that the hydrophilic coating equilibrates completely with the RH of the external atmosphere, with no moisture concentration gradients within the film. Film permeability is a function of moisture concentration, which is related to the RH of the surrounding air. Thus, film permeability would be constant throughout the film with our assumption. This assumption may be valid if the fruit skin resistance to water vapor is high and the film resistance to water vapor is low. In this way, the moisture concentration within the film would be similar to that of the coating surface facing the atmosphere. However, fruits with low skin resistance to water vapor may generate large gradients of moisture concentration within the coating. For example, fruits with low skin resistance that are exposed to low RH may lose moisture readily, exposing the inner fruit coating surface to large moisture concentration values and the coating air interface to smaller moisture concentration values. If this occurs, then the average coating permeability would be in between the permeability values for the conditions at the fruit coating and the coating air surfaces, depending on the moisture sorption isotherm of the biopolymer. In the extreme case of fresh-cut surfaces of fruits (Baldwin and others 1995), coatings facing the fruit coating surface will most likely be exposed to a water activity of about 1 (approximately 100% RH). This will indeed increase the overall coating permeability and reduce its barrier properties. Thus, we propose that similar coatings applied to different fruit surfaces may have quite different barrier performances. It was also assumed that there is no boundary layer resistance to water vapor transmission. The existence of a boundary layer would increase the moisture concentration of the coating because of an increase in the surrounding water vapor pressure adjacent to the film air surface. This increase in film moisture concentration will increase the overall coating gas permeability affecting the internal gas composition. The assumption of a negligible boundary layer will hold if the air velocity of the surrounding atmosphere is high enough to reduce it. In commercial storage conditions, as well as in research, this assumption is not always met. Another source of variation is the RH gradients experienced in commercial storage facilities, as well as in research techniques used for controlling RH. In the latter, it is common to adjust the RH of the air inlet in-flow-through systems when measuring respiration of fruits in jars. However, large RH gradients may still occur inside the jars, depending on the speed and RH of the air inlet, the amount of fruit in the jar, and the fruit water loss rate. Other sources of variation to consider are temperature fluctuations and film thickness variations. Changes in temperature may affect the fruit V m, the film gas permeability, and the external RH conditions. Film thickness may vary on individual fruits, changing the total gas resistance of the fruit. In summary, the predicted trends obtained by varying RH conditions support the idea that film coating permeability and coating thickness are important in modified atmospheres of coated fruits, following either a lac model or a tac model for the specific case of complete pore coverage. However, if incomplete pore blockage is achieved, then film coating permeability and thickness may not be important in modifying the atmosphere within the fruit (Banks and others 1993). Thus, for the incomplete pore blockage model, the RH would not have a major effect in the MA of coated fruits. Conclusions IT IS POSSIBLE, WHEN USING THE APPROACH DESCRIBED IN THIS article, to observe and understand the interaction of storage conditions (such as RH) and the composition of film coatings applied to a fruit surface. This analysis can explain, in part, the variability of results observed when coatings are applied on a commercial scale and in research. The approach supports the idea that film coating thickness and permeability play an important role in internal gas modification of coated fruits. Therefore, it is necessary to understand the physicochemical properties of the materials used as edible coatings and their moisture sorption behavior. Using this approach will make it possible to predict coating performance once applied to a real fruit system and to tailor appropriate coatings to achieve a target gas composition. References Baldwin E, Nisperos-Carriedo M, Baker R Edible coatings for lightly processed fruits and vegetables. HortScience 30: Banks N, Cutting J, Nicholson S Approaches to optimizing surface coatings for fruits. New Zealand J Crop Hort Sci 25: Banks N, Dadzie B, Cleland D Reducing gas exchange of fruits with surface coatings. Postharvest Biol Technol 3: Ben-Yehoshua S, Cameron A Exchange determination of water vapor carbon dioxide, oxygen, ethylene and other gases of fruits and vegetables. In: Linskens HF, Jackson JF, editors. Gases in plant and microbial cells. Modern methods of plant analysis. New series 9. p (Brusewitz and Singh 1985?) Buera P, Karel M Application of the WLF equation to describe the combined effects of moisture and temperature on non-enzymatic browning rates in food systems. J Food Proc Preserv 17: Elson C, Hayes E, Lidster P Development of the differentially permeable fruit coating Nutri-Save for the modified atmosphere storage of fruit. 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6 Coated fruits and vegetables and RH effects Roos Y, Karel M Applying state diagrams to food processing and development. Food Technol 45:66-71, 107. Roth W, Mehltretter C Some properties of hydroxypropylated amylomaize starch films. Food Technol 21: Shellhammer T, Krochta J Whey protein emulsion film performance as affected by lipid type and amount. J Food Sci 62: (Trout and others 1953?) MS Submitted 8/17/01, Accepted 3/16/02, Received 4/17/02 Author Cisneros-Zevallos is with the Dept. of Horticultural Sciences, Texas A&M Univ., College Station, TX Author Krochta is with the Dept. of Food Science & Technology, Univ. of California, Davis, CA Direct inquiries to author Cisneros-Zevallos ( lcisnero@tamu.edu). Concise Reviews in Food Science Vol. 67, Nr. 6, 2002 JOURNAL OF FOOD SCIENCE 1995